2.3. Preparation of standards
The aflatoxins B1, B2, G1 and G2 standards (1.0 mg of each aflatoxin in capped amber bottles) were purchased from Sigma–Aldrich.The working solutions were prepared according to the AOAC procedure (AOAC, 2000) by injecting 1 mL of acetonitrile into each vial to dissolve the aflatoxins. Working solutions (50–1000 ng g1) were diluted in acetonitrile and stored at 8 C. They were evaporated to dryness under nitrogen before being derivatised with TFA according to the procedure describe below.